1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight unit including a light emitting diode (LED) and an LCD device on which the backlight unit is mounted.
2. Discussion of the Related Art
A Cathode Ray Tube (CRT), one of flat display devices, has been widely used for monitors of devices such as a television, a measuring machine and an information terminal. However, the CRT has limitations to miniaturization and lightness in weight due to its big size and heavy weight. Accordingly, display devices such as a liquid crystal display (LCD) device using an electro-optics effect, a plasma display panel (PDP) using a gas discharge and an Electroluminescence display (ELD) device using an electro-luminescence effect have been actively studied, which can be the substitutes for the CRT.
Among the display devices, the LCD device is most actively studied. The LCD device having low power consumption, thin profile and lightness in weight is highly developed for being applied to monitors for a desktop computer and a large sized display device as well as for a laptop computer. Accordingly, demands for the LCD devices continuously increase.
The LCD device includes an LCD panel for displaying a picture image, a driving part for applying a driving signal to the LCD panel, and a backlight unit for providing a light source to the LCD panel. The LCD panel has first and second glass substrates bonded to each other at a predetermined interval, and a liquid crystal layer formed between the first and second glass substrates.
On the first glass substrate (TFT array substrate), there are a plurality of gate lines arranged in a first direction at fixed intervals, a plurality of data lines arranged in a second direction in perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in respective pixel regions defined by the gate lines and the data lines in a matrix type, and a plurality of thin film transistors (TFTs) switchable in response to signals on the gate lines for transmission of signals on the data lines to the pixel electrodes.
The second glass substrate (color filter substrate) has a black matrix layer for shielding light from areas excluding the pixel regions, a color filter layer (R, G, B) for displaying colors, and a common electrode for implementing a picture image.
The first glass substrate is positioned at the predetermined interval from the second glass substrate by spacers. The first and second glass substrates are bonded to each other by a sealant having a liquid crystal injection inlet. Then, liquid crystal is injected through the liquid crystal injection inlet.
Meanwhile, the LCD device controls transmittance of ambient light to display the picture image. In this respect, the LCD device requires an additional light source such as a backlight. The backlight is classified into a direct type and an edge type according to the arrangement of lamps.
The LCD device uses the light source such as an Electro Luminescence (EL), a Light Emitting Diode (LED), a Cold Cathode Fluorescent Lamp (CCFL) or a Hot Cathode Fluorescent Lamp (HCFL). Among those light sources, the CCFL having long lifetime, low power consumption and thin profile is used as the light source for a large sized color TFT LCD device.
In case of the CCFL as the light source, a fluorescent discharge tube is used for a penning effect, which is formed by injecting a hydrargyrum gas containing Argon (Ar) and Neon (Ne) at a low temperature. Also, electrodes are formed at both ends of the fluorescent discharge tube, and the cathode is formed in a plate-shape. When a voltage is applied thereto, electric charges inside the fluorescent discharge tube collide against the plate-shaped cathode like a sputtering state, thereby generating secondary electrons. Thus, circumferential elements are excited by the secondary electrons, whereby plasma is generated. Also, the circumferential elements emit strong ultraviolet rays, and then the ultraviolet rays excite a fluorescent substance, thereby emitting visible rays.
In the edge type backlight, a lamp unit is formed at one side of a light-guiding plate. Generally, the edge type is applied to relatively small sized LCD devices such as the monitors for the laptop computer or the desktop computer. The edge type backlight is useful to obtain uniform luminance, long lifetime and thin profile in the LCD device.
Hereinafter, a related art backlight unit will be described with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view of an edge type backlight unit including a fluorescent lamp according to the related art.
As shown in FIG. 1, a related art edge type backlight unit includes a fluorescent lamp 11, a lamp housing 12, a light-guiding plate 13, a reflecting sheet 14, a light-diffusion plate 15, a prism sheet 16, a protection sheet 17, and a main supporter 18. A fluorescent substance is coated on an inner surface of the fluorescent lamp 11 for emitting light. Also, the lamp housing 12 fixes the fluorescent lamp 11, and concentrates the light emitted from the fluorescent lamp 11 to one direction. The light-guiding plate 13 provides the light emitted from the fluorescent lamp 11 to an upper side of an LCD panel, and the reflecting sheet 14 is provided below the light-guiding plate 13 to guide the light leaking in an opposite side of the LCD panel toward the light-guiding plate 13. The light-diffusion plate 15 is formed over the light-guiding plate 13 to uniformly diffuse the light guided by the light-guiding plate 13. Also, the prism sheet 16 is formed over the light-diffusion plate 15 to concentrate the light diffused in the light-diffusion plate 15, and to transmit the concentrated light to the LCD panel. The protection sheet 17 is formed on an upper side of the prism sheet 16 to protect the prism sheet 16. The main supporter 18 receives and fixes the aforementioned elements.
In the aforementioned backlight unit, the light emitted from the fluorescent lamp 11 is concentrated to an incident surface of the light-guiding plate 13, and then the concentrated light passes through the light-guiding plate 13, the light-diffusion plate 15 and the prism sheet 16 in sequence, whereby the light is transmitted to the LCD panel. However, the backlight unit of using the related art fluorescent lamp has a low color realization ratio due to the emission characteristics of the light source. Furthermore, it is hard to obtain the backlight unit having high luminance due to limits in size and capacity of the fluorescent lamp.
With trend of the large-sized LCD device of 20-inch or more, the direct type backlight unit is actively developed, in which a plurality of lamps are provided in lines on a lower surface of a light-diffusion plate, whereby an entire surface of an LCD panel is directly illuminated with light. The direct type backlight unit, which has greater light efficiency as compared with that of the edge type backlight unit, is used for the large-sized LCD device requiring high luminance.
In the meantime, the edge type and direct type backlight units use the fluorescent lamp as the light source. However, the fluorescent lamp is problematic in that it requires a noxious gas. That is, the fluorescent lamp may cause the environmental contamination. Thus, new light sources have been researched and studied recently, which can prevent the environmental contamination. Among the various light sources newly developed, an LED (Light Emitting Diode) draws great attentions in that it has no environmental contamination, it can display various colors, and it can decrease the power consumption.
FIG. 2 is a plane view of a backlight unit including an LED according to the related art. FIG. 3 is a cross sectional view of an LCD device, on which a backlight unit is mounted, along I-I′ of FIG. 2. As shown in FIG. 2, a backlight unit including an LED according to the related art includes a cover bottom 20 (shown in FIG. 3), a plurality of metal PCBs (Printed Circuit Board) 75, a plurality of red R, green G and blue B LEDs 70a, 70b and 70c, and a plurality of optical sheets 50 (shown in FIG. 3).
The metal PCBs 75 are formed in parallel to one another on the cover bottom 20. Then, the red R, green G and blue B LEDs 70a, 70b and 70c are arranged at fixed intervals on each of the metal PCBs 75. Also, the optical sheets 50 are formed on the red R, green G and blue B LEDs 70a, 70b and 70c. 
The backlight unit including the LEDs, shown in FIG. 2, is formed in a direct type, in which the LEDs 70a, 70b and 70c are formed below the LCD panel 40 (shown in FIG. 3). As shown in FIG. 3, the LCD device, which includes the backlight unit comprising of the LEDs according to the related art, includes the cover bottom 20, the metal PCBs 75, the red R, green G and blue B LEDs 70a, 70b and 70c, the optical sheets 50, the LCD panel 40, a guide panel 60, and a case top 30.
The metal PCBs 75 are formed on the cover bottom 20, wherein the metal PCBs 75 are formed in parallel. Then, the red R, green G and blue B LEDs 70a, 70b and 70c are formed at fixed intervals on each of the metal PCBs 75. Then, the optical sheets 50 are formed on the LEDs 70. The LCD panel 40 is formed on the optical sheets 50. The guide panel 60 supports the LCD panel 40 and the optical sheets 50. Also, the case top 30 is provided at an upper edge of the LCD panel 40 and lateral sides of the guide panel 60 and the cover bottom 20. Each of the LEDs 70 includes a body 73 and a light-emission part 71. A reflecting sheet 25 is formed between the body 73 and the light-emission part 71.
The guide panel 60 has a protruding pattern. The protruding pattern of the guide panel 60 is positioned between the optical sheets 50 and the LCD panel 40, whereby the predetermined interval is formed between the optical sheets 50 and the LCD panel 40. Also, the cover bottom 20 is positioned at the predetermined interval from the lower surface of the optical sheets 50. Thus, it is possible to provide a space for receiving the metal PCBs 75 and the LEDs 70 between the cover bottom 20 and the optical sheets 50. The metal PCBs 75 are formed at fixed intervals. The metal PCBs 75 help the discharge of heat emitted from the LEDs 70.
The LCD panel 40 includes lower and upper substrates bonded to each other at the predetermined interval, a liquid crystal layer (not shown) formed between the lower and upper substrates, and lower and upper polarizing sheets respectively formed on outer surfaces of the lower and upper substrates. The optical sheets 50 are formed of first and second prism sheets, and a light-diffusion sheet.
For displaying images on the LCD panel 40, the backlight unit turns on the LEDs 70. In this case, a voltage may be applied to all of the red R, green G and blue B LEDs 70a, 70b and 70c, or may be selectively applied to the red R, green B and blue B LEDs 70a, 70b and 70c. Thus, the light emitted from the red R, green G and blue B LEDs is color-mixed in the space between the optical sheets 50 and the LEDs 70, whereby the rear surface of the LCD panel 40 is irradiated with the mixed light.
In the backlight unit including the LEDs, the LEDs 70 are mounted on each of the metal PCBs 75, and each of the metal PCB 75 is connected with the cover bottom 20. In comparison with a CCFL (Cold Cathode Fluorescent Lamp), the LED 70 has the lower light efficiency. Thus, to obtain the desirable luminance on the entire surface of the LCD panel, it is necessary to increase the power consumption. That is, the LED 70 generates more heat than the CCFL.
In the aforementioned structure, the heat generated from the LED 70 is mostly conducted and diffused to the metal PCB 75, and is then discharged to the outside through the cover bottom 20. In this case, the metal PCB 75 is formed of aluminum, and the cover bottom 20 is formed of aluminum, aluminum alloy, or MCPET (Micro Polyethyene Ether-phthalein).
The aluminum or aluminum alloy has a thermal conduction coefficient below 100 W/mK, and MCPET has a thermal conduction coefficient of 0.2 W/mK. Thus, it is impossible to discharge the heat generated from the LEDs 70 to the outside in a short time. Also, the heat generated from the LEDs 70 is discharged to the outside through the lower side (PCB and cover bottom) of the backlight unit. That is, only small amount of heat is discharged through the lower side of the backlight unit. Therefore, it is impossible to achieve the thermal reliability of LED in operation of the LCD device.
That is, the backlight unit including the LEDs according to the related art has the following disadvantages.
In the backlight unit according to the related art, the LEDs are positioned on each of the PCBs, wherein the PCBs are formed of aluminum. That is, the heat generated from the LEDs is discharged to the outside through the PCBs. If driving the backlight unit for a long time, it is impossible to discharge the heat generated from the LEDs to the outside of the backlight unit in a short time due to the limitation in thermal conduction of aluminum.
Also, the heat generated from the LEDs is discharged to the outside through the PCBs and the cover bottom. Therefore, it is impossible to discharge the generated heat to the outside in a short time.
Furthermore, the heat is mostly generated in the LED chip. However, the LED chip has no space for discharging the heat since the reflecting sheet is formed in correspondence with the LED chip. Therefore, it is difficult to discharge the generated heat to the outside in a short time. Accordingly, the temperature increases in the junction between each of the LEDs, thereby deteriorating the reliability in operation of the LED.